The sill plate there on 28/29 roadsters attach to the sub rail to strengthen the area and add rigidity there. 30 & 31 are different but not a lot. I see no point in arguing about nomenclature. To me it is substructure and it is a rail in the way it is shaped. Most folks that sell these items refer to them as sub rails but they also sell sill plates.
https://www.snydersantiqueauto.com/P...arts/sub-rails